1. Field of the Invention
The present invention relates to an imaging optical system which can be provided in an imaging apparatus such as, e.g., an in-vehicle camera, a surveillance camera, and a portable terminal (including a mobile phone, a smart phone or other smart devices).
2. Description of Related Art
An imaging optical system is known in the related art that can achieve both a wide angle-of-view and a higher definition of a telescopic image in a central area of a picture frame. Patent Literature 1 discloses an imaging optical system which achieves a wide angle-of-view by causing a large amount of negative distortion, and achieves an enlarged (and distorted) image in the central area of the picture frame.
Patent Literature 1: Japanese Unexamined Patent Publication No. 2005-010521.
However, in the imaging optical system of Patent Literature 1, the overall length of the imaging optical system becomes long when set to a large aperture; and if the overall length of the imaging optical system is shortened, there is the disadvantage of not being able to provide a large aperture ratio. For example, the fourth embodiment in Patent Literature 1 discloses an imaging optical system having a relatively large aperture having an f-number of about 1.8. However, the value of the overall length of the imaging optical system divided by the maximum image height is 9.8; hence, the overall length of the imaging optical system is long relative to the maximum image height. Whereas, in the first embodiment of Patent Literature 1, the value of the overall length of the imaging optical system divided by the maximum image height is 5.1, so that the overall length of the imaging optical system is short relative to the maximum image height; however, the f-number is 2.8, i.e., the aperture ratio is not very large.